160 On Gold Refining oy Chlorine Gas. 



only on the silver thus added, but also on that originally con- 

 tained in the gold. To arrive at this end, a complicated and very 

 costly plant is required, besides large quantities of expensive 

 acids, and several days are required in the operation. It is 

 evident, then, that if all this complicated process can be avoided, 

 and the silver simply and completely separated in one operation 

 at the time the gold is being melted, a very great saving of time, 

 of material, of plant, and of the interest involved in all these 

 will be effected. 



Such an end is attained in the plan now being adopted for 

 effecting this operation. 



It is well-known that chlorine readily enters into combination 

 with almost every known metal, the action in some cases being 

 so violent as to be attended with vivid combustion. Many 

 metals, such as lead, tin, zinc, and antimony, when introduced 

 into this gas, even at ordinary temperatures, combine with it, 

 forming highly volatile chlorides. The two latter, if in a state of 

 fine division, burst into flame on being placed in an atmosphere 

 of chlorine. 



Copper also exhibits spontaneous combustion under similar 

 circumstances, but the resulting chloride formed is only slightly 

 volatile. 



Silver immersed in chlorine gas at ordinary temperatures 

 slowly unites with it, forming chloride of silver ; but if the gas 

 be passed over it while red hot, the action is much more ener- 

 getic, the compound formed being more volatile than the chloride 

 of copper, but much less so than those of lead, tin, zinc, or 

 antimony. 



The method of refining now to be described is based upon 

 these facts. 



It consists simply in passing a current of chlorine gas through 

 the gold, while in a melted state, which is easily done by thrusting 

 into the molten metal a small clay tube connected with a stone- 

 ware vessel in which chlorine is generated. 



The chlorine, on coming in contact with silver in the molten 

 alloy, at once combines with it, forming chloride of silver, which, 

 being of less specific gravity, rises to the surface of the melted 

 go d, while the latter remains in a purified condition beneath. 



Chloride of silver has always been considered a somewhat 

 volatile substance, and under circumstances such as those here 

 described it was naturally Mipposed that it would either be 

 sublimed in the flue, or escape entirely up the chimney, but in 

 practice it is found that the volatility of the chloride is not nearly 

 so great as might have been anticipated, and that if its surface 

 is coated with a layer of fused borax, it may be kept melted at a 

 high temperature without any very material loss. 



The furnace required for the operation is the ordinary 12-inch 



